Epoxy resin composition, prepreg, and fiber-reinforced composite material

ABSTRACT

The present invention provides an epoxy resin composition that serves to produce a cured epoxy resin that simultaneously realizes a high heat resistance, a high elastic modulus, and a low color and to produce a molded article having a good appearance without suffering the formation of white spots on the surface thereof when used as matrix resin in a fiber reinforced composite material. It provides an epoxy resin composition that includes an epoxy resin as component [A] and an imidazole compound as component [B] and meets the undermentioned conditions (a) to (d): (a) it contains an isocyanurate type epoxy resin [A1] as component [A] in an amount of 10 to 40 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (b) it contains a bisphenol type epoxy resin [A2] as a component [A] in an amount of 40 to 90 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (c) resin [A2] has an average epoxy equivalent weight of 220 to 500 g/eq, and (d) the content of component [B] is such that the ratio of the number of imidazole groups to the total number of epoxy groups in the epoxy resins is 0.01 to 0.06.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2017/023188, filed Jun. 23, 2017, which claims priority to Japanese Patent Application No. 2016-127268, filed Jun. 28, 2016, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to an epoxy resin composition adapted for use as matrix resin of a fiber reinforced composite material suitable for sport applications and general industry applications, and also relates to a prepreg and a fiber reinforced composite material prepared by using such an epoxy resin composition as matrix resin.

BACKGROUND OF THE INVENTION

Epoxy resins have been widely used in various industries as a coating composition, adhesive, electric and electronic information material, advanced composite material, or the like because of their excellent mechanical properties. Epoxy resins have been frequently used particularly in fiber reinforced composite materials composed mainly of a reinforcing fiber, such as carbon fiber, glass fiber, and aramid fiber, combined with a matrix resin.

The use of a prepreg produced by impregnating a carbon fiber base with an epoxy resin is popular in the production of carbon fiber reinforced composite materials. Such a prepreg is laminated in layers or preformed, and then heated to cure the epoxy resin, thereby producing a molded article. If the curing reaction proceeds before the lamination step, the prepreg will suffer deterioration in handling property. Therefore, an epoxy resin to use in a prepreg requires high preservation stability, and dicyandiamide is widely used as curing agent because it is high in latent curing property.

Being lightweight and having high strength and high stiffness, carbon fiber composite materials have been used in a variety of fields ranging from sport and leisure goods to industrial applications such as automobiles and aircraft. With this feature, they have been frequently used in recent years not only in structure members, but for texture-of-cloth decoration realized by arranging woven fabrics in surfaces. For an epoxy resin adopted as matrix resin, therefore, importance is now attached to the low colors of cured products and appearance of molded articles in addition to high heat resistance and good mechanical properties of cured products. If dicyandiamide is used as hardener, however, there occurs the problem of white spots being formed on the surface of the resulting molded article, leading to deterioration in the appearance thereof.

As a method to prevent the formation of white spots attributed to dicyandiamide, Patent document 1 discloses a technique designed to depress the formation of white spots in prepreg by adopting a masterbatch containing dicyandiamide particles with small diameters to allow the dicyandiamide and epoxy resin to be dissolved or compatibilized during the step for impregnating the base. As methods to eliminate the use of dicyandiamide, Patent document 2 discloses a technique that uses polythiol and a urea compound as hardener components and Patent document 3 discloses a technique that employs an acid anhydride as hardener.

PATENT DOCUMENTS

-   Patent document 1: Japanese Unexamined Patent Publication (Kokai)     No. HEI 11-209580 -   Patent document 2: Japanese Unexamined Patent Publication (Kokai)     No. 2013-253194 -   Patent document 3: Japanese Unexamined Patent Publication (Kokai)     No. 2013-133407

SUMMARY OF THE INVENTION

In the case of the method described in Patent document 1, however, dicyandiamide is dissolved or compatibilized during the prepreg production step and as a result, the prepreg obtained fails to realize a sufficiently high preservation stability when used in a fiber reinforced composite material. In the case where dicyandiamide is not dissolved in the prepreg production step, the resulting molded articles occasionally suffer the formation of white spots on the surfaces thereof.

In the case of the method described in Patent document 2, the resulting molded articles do not suffer the formation of white spots on the surfaces thereof because dicyandiamide is not present, but the resulting cured resin products occasionally fail to have sufficiently high heat resistance or good mechanical properties.

In the case of using an acid anhydride hardener as proposed in Patent document 3, the resulting molded articles are free of white spots on the surfaces thereof, but the acid anhydride used as the hardener is occasionally degraded by the moisture in air, possibly leading to a cured resin product with deteriorated physical properties. Accordingly, it is not preferred to apply this method to the production of a prepreg for a fiber reinforced composite material that is expected to be stored for a certain period.

An object of the present invention is to eliminate the drawbacks of these conventional techniques to provide an epoxy resin composition that serves to produce a cured epoxy resin simultaneously realizing a high heat resistance, a high elastic modulus, and a low color and to produce a molded article having a good appearance without suffering the formation of white spots on the surface thereof when used as matrix resin in a fiber reinforced composite material, and also provide a prepreg produced from the epoxy resin composition, and a fiber reinforced composite material that is obtained by curing the prepreg and that suffers no white spots on the surface thereof and has a good appearance.

After making an intensive study aiming to solve the problems described above, the inventors of the present invention found an epoxy resin composition having the following constitution and arrived at the present invention on the basis of the finding. More specifically, the epoxy resin composition according to the present invention has the constitution described below.

An epoxy resin composition containing epoxy resins as component [A] and an imidazole compound as component [B] and meeting the undermentioned conditions (a) to (d):

(a) it contains an isocyanurate type epoxy resin [A1] as component [A] in an amount of 10 to 40 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (b) it contains an bisphenol type epoxy resin [A2] as a component [A] in an amount of 40 to 90 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (c) resin [A2] has an average epoxy equivalent weight of 220 to 500 g/eq, and (d) the content of component [B] is such that the ratio of the number of imidazole groups to the total number of epoxy groups in the epoxy resins is 0.01 to 0.06.

The prepreg according to the present invention is a prepreg that includes the aforementioned epoxy resin composition and reinforcing fiber.

Furthermore, the fiber reinforced composite material according to the present invention is a fiber reinforced composite material that is obtainable by curing the aforementioned prepreg.

The present invention can provide an epoxy resin composition that serves to produce a cured epoxy resin realizing a high heat resistance, good mechanical properties and a low color and to produce a molded article having a good appearance without suffering the formation of white spots on the surface thereof when used as matrix resin in a fiber reinforced composite material.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The epoxy resin composition according to the present invention contains an epoxy resin as component [A] and an imidazole compound as component [B], both being essential components.

<Component [A]>

Component [A] for the present invention consists of epoxy resins. Examples thereof include glycidyl ether type epoxy resins such as bisphenol type epoxy resins, biphenyl type epoxy resins, naphthalene type epoxy resins, novolac type epoxy resins, epoxy resins having fluorene backbones, epoxy resins formed from copolymers of a phenol compound and dicyclopentadiene, diglycidyl resorcinol, tetrakis(glycidyloxyphenyl)ethane, and tris(glycidyloxyphenyl)methane; and glycidylamine type epoxy resins such as tetraglycidyl diaminodiphenylmethane, triglycidyl aminophenol, triglycidyl aminocresol, and tetraglycidyl xylene diamine.

For the present invention, an isocyanurate type epoxy resin [A1] is contained as component [A].

As component [A1] is contained, it serves to produce a cured resin product having a high elastic modulus and also having an improved heat resistance, thus allowing the production of a fiber reinforced composite material having good mechanical properties and a high heat resistance.

It is essential for component [A1] to account for 10 to 40 parts by mass relative to the total quantity of epoxy resins, which accounts for 100 parts by mass, in the epoxy resin composition, and it is preferable that the lower limit is 15 parts by mass or more and that the upper limit is 30 parts by mass or less. As component [A1] is contained in an amount in this range, it serves to produce a cured resin product having a low color and also having an elastic modulus and a heat resistance in a good balance.

Examples of useful commercial products of component [A1] include TEPIC (registered trademark) -S, -L, -PAS B22 (all manufactured by Nissan Chemical Industries, Ltd.) and Araldite (registered trademark) PT9810 (manufactured by Huntsman Advanced Materials Gmbh).

For the present invention, a bisphenol type epoxy resin [A2] is contained as component [A]. As resin [A2] is contained, it serves to produce a cured resin product having a decreased color and a fiber reinforced composite material having a good appearance.

It is essential for resin [A2] to account for 40 to 90 parts by mass relative to the total quantity of epoxy resins, which accounts for 100 parts by mass, in the epoxy resin composition, and it is preferable that the lower limit is 70 parts by mass or more and that the upper limit is 90 parts by mass or less. If resin [A2] is contained in an amount in this range, it serves to produce a cured resin product having a color and an elastic modulus in a good balance.

Examples of resin [A2] include epoxy resins produced by glycidyl-etherification of bisphenol compounds such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol AD type epoxy resin, and bisphenol S type epoxy resin.

Examples of useful commercial products of bisphenol A type epoxy resin include jER (registered trademark) 825, 828, 834, 1001, 1002, 1003, 1003F, 1004, 1004AF, 1005F, 1006FS, 1007, 1009, and 1010 (all manufactured by Mitsubishi Chemical Corporation).

Examples of useful commercial products of bisphenol F type epoxy resin include jER (registered trademark) 806, 807, 4002P, 4004P, 4007P, 4009P, and 4010P (all manufactured by Mitsubishi Chemical Corporation), Epotohto (registered trademark) YDF2001 and YDF2004 (both manufactured by Nippon Steel Chemical Co., Ltd.), and EPICRON (registered trademark) 830, 830-S, and 835 (all manufactured by DIC Corporation).

Examples of useful commercial products of bisphenol S type epoxy resin include EPICRON (registered trademark) EXA-1514 (manufactured by DIC Corporation).

For the present invention, it is essential for resin [A2] in the epoxy resin composition to have an average epoxy equivalent weight of 220 to 500 g/eq from the viewpoint of the balance between the heat resistance and the color of the cured epoxy resin and it is preferable that the lower limit is 300 g/eq or more and that the upper limit is 400 g/eq or less. If the average epoxy equivalent weight of resin [A2] is less than 220 g/eq, the resulting cured resin will have a decreased heat resistance and increased coloring, possibly leading to a fiber reinforced composite material with a poor appearance. On the other hand, if the average epoxy equivalent weight of resin [A2] is more than 500 g/eq, the coloring will be weak, but the resulting cured resin will have a decreased heat resistance.

The average epoxy equivalent weight of resin [A2] in the epoxy resin composition can be calculated by the method described below.

(Method for Calculation of the Average Epoxy Equivalent Weight of Resin [A2] in the Epoxy Resin Composition)

In the case of an epoxy resin composition containing a combination of n types of epoxy resins as resin [A2], with the epoxy resins in resin [A2] accounting for G′ parts by mass in total and each epoxy resin X, which has an epoxy equivalent weight of Ex (g/eq), in resin [A2] accounting for Wx parts by mass, the average epoxy equivalent weight (g/eq) of resin [A2] is calculated by Equation (I) (where x=1, 2, 3, . . . , n) given below.

$\begin{matrix} {\mspace{79mu} \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack} & \; \\ {{{Average}\mspace{14mu} {epoxy}\mspace{14mu} {equivalent}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {{resin}\mspace{14mu}\left\lbrack {A\; 2} \right\rbrack}\mspace{14mu} {in}\mspace{14mu} {epoxy}\mspace{14mu} {resin}\mspace{14mu} {composition}\mspace{14mu} \left( {g\text{/}{eq}} \right)} = \frac{G^{\prime}}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & (I) \end{matrix}$

<Component [B]>

For the present invention, component [B] is an imidazole compound. For the present invention, the imidazole compound in component [B] works as a hardener to promote the self-polymerization of the epoxy resins in component [A]. The use of an imidazole compound serves to produce a cured epoxy resin having a low color and a good balance with heat resistance in comparison with other self-polymerization type hardeners.

For the present invention, the content of component [B] is such that the ratio of the number of imidazole groups to the total number of epoxy groups in the epoxy resins (i.e., the molar ratio of imidazole to the epoxy groups in all epoxy resins) is 0.01 to 0.06 from the viewpoint of the balance between the heat resistance and the color of the cured epoxy resin, and it is preferable that the lower limit is 0.015 or more and that the upper limit is 0.05 or less. If the content of component [B] is so small that the ratio of the number of imidazole groups to the number of epoxy groups is less than 0.01, it leads to a cured resin with a decreased heat resistance. If the content of component [B] is so large that the ratio of the number of imidazole groups to the number of epoxy groups is more than 0.06, it leads to a cured resin with a high color and accordingly leads to a fiber reinforced composite material with a poor appearance.

The aforementioned ratio of the number of imidazole groups to the number of epoxy groups is calculated by the method described below.

(Method for Calculation of Ratio of the Number of Imidazole Groups to the Number of Epoxy Groups in Epoxy Resin Composition)

(1) Calculation of average epoxy equivalent weight over all epoxy resins

In the case of an epoxy resin composition containing a combination of n types of epoxy resins as component [A], with the epoxy resins accounting for G parts by mass in total and each epoxy resin Y, which has an epoxy equivalent weight of Ey (g/eq), accounting for Wy parts by mass, the average epoxy equivalent weight (g/eq) over all epoxy resins is calculated by Equation (II) (where y=1, 2, 3, . . . , n) given below.

$\begin{matrix} {\mspace{79mu} \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack} & \; \\ {{{Average}\mspace{14mu} {epoxy}\mspace{14mu} {equivalent}\mspace{14mu} {weight}\mspace{14mu} {over}\mspace{14mu} {all}\mspace{14mu} {epoxy}\mspace{14mu} {{resin}s}\mspace{25mu} {in}\mspace{14mu} {epoxy}\mspace{14mu} {resin}\mspace{14mu} {composition}\mspace{14mu} \left( {g\text{/}{eq}} \right)} = \frac{G}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & ({II}) \end{matrix}$

(2) Calculation of the ratio of the number of imidazole groups to the number of epoxy groups In the case of an epoxy resin composition containing epoxy resins with a total quantity accounting for G parts by mass and also containing an imidazole compound accounting for W parts by mass in the epoxy resin composition and having an imidazole equivalent weight of I (g/eq), the ratio of the number of imidazole groups to the number of epoxy groups in the epoxy resin composition is calculated by Equation (III) from the value obtained in the above paragraph (1).

[Formula 3]

Ratio of the number of imidazole groups to the number of epoxy groups in epoxy resin composition=average epoxy equivalent weight of epoxy resin composition×W/(G×I)  (III)

Examples of the imidazole compounds include 1-benzyl-2-methyl imidazole, 1-benzyl-2-ethyl imidazole, 1-cyanoethyl-2-methyl imidazole, 1-cyanoethyl-2-ethyl-4-methyl imidazole, and 1-cyanoethyl-2-phenyl imidazole. These imidazole compounds may be used singly or as a combination of two or more thereof. When a plurality of imidazole compounds are used in combination, the ratio of the number of imidazole groups to the number of epoxy groups is determined by calculating the average imidazole equivalent weight as in the case of the average epoxy equivalent weight and using it as the aforementioned imidazole equivalent weight. Hereinafter, both the imidazole equivalent weight for the case where an imidazole compound is used singly and the average imidazole equivalent weight for the case where a plurality of imidazole compounds are used in combination are referred to collectively as the imidazole equivalent weight of component [B].

It is preferable for component [B] to have an imidazole equivalent weight of 180 g/eq or more. If component [B] has an imidazole equivalent weight of 180 g/eq or more, it will tend to result in a cured epoxy resin with a reduced color and an improved heat resistance, making it possible to easily produce a fiber reinforced composite material having a good appearance and a high heat resistance. The upper limit of the imidazole equivalent weight is preferably 1,000 g/eq or less.

From the viewpoint of increasing the imidazole equivalent weight, it is preferable that a compound [B1] as described by the undermentioned general formula (I) is contained as component [B].

(In the formula, R₁, R₂, R₃ and R₄ are each independently a hydrogen atom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, or a phenyl group, and X is an alkylene group or an aromatic hydrocarbon group.)

Compound [B1] is an addition product obtainable through a reaction between an imidazole compound and an isocyanate compound. Commercial products of the addition product include G-8009L (manufactured by DKS Co. Ltd.).

Similarly, from the viewpoint of increasing the imidazole equivalent weight, furthermore, it is also preferable that a compound [B2] as represented by the undermentioned general formula (II) is contained as component [B].

(In the formula, R₅, R₆, R₇, and R₈ are each independently a hydrogen atom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, or a phenyl group, and Y is a single bond, an alkylene group, an alkylidene group, an ether group, or a sulfonyl group).

Compound [B2] is an addition product obtainable through a reaction between an imidazole compound and an epoxy compound. Commercial products of the addition product include Cureduct (registered trademark) P-0505 (Shikoku Chemicals Corporation) and JER cure (registered trademark) P200H50 (Mitsubishi Chemical Corporation).

The incorporation of a dicyandiamide as hardener may result in a molded article suffering the formation of white spots on the surface thereof to deteriorate the appearance. For the present invention, therefore, it is preferable for the dicyandiamide to account for 0.5 part by mass or less, more preferably 0.2 part by mass or less, relative to the total quantity of epoxy resins, which accounts for 100 parts by mass, and it is most preferable that dicyandiamide is not contained.

<Component [C]>

An acidic compound may be added to as component [C] the epoxy resin composition according to the present invention. For the present invention, the acidic compound of component [C] works as a stabilization agent for the imidazole compound of component [B]. The addition of an acidic compound is preferred in order to provide an epoxy resin composition or prepreg with an improved preservation stability.

Examples of the acidic compound include Broensted acid and Lewis acid.

Preferred examples of the Broensted acid include various carboxylic acids. The carboxylic acids can be categorized into aliphatic monocarboxylic acids, aromatic monocarboxylic acids, aliphatic polycarboxylic acids, and aromatic polycarboxylic acids, and include, for example, the following compounds.

Examples of aliphatic monocarboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, octyl acid, pelargonic acid, lauryl acid, myristic acid, stearic acid, behenic acid, undecane acid, acrylic acid, methacrylic acid, crotonic acid, oleic acid, and derivatives thereof.

Examples of aliphatic polycarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, tetradecanedioic acid, pentadecanedioic acid, and derivatives thereof.

Examples of aromatic monocarboxylic acids include benzoic acid, cinnamic acid, naphthoic acid, toluic acid, and derivatives thereof.

Examples of aromatic polycarboxylic acids include phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, pyromellitic acid, and derivatives thereof.

Of these Broensted acids, it is preferable to use an aromatic monocarboxylic acid, particularly preferably benzoic acid, as compound [C1].

As a Lewis acid, the use of boric acid and/or a borate ester compound or the like is desirable.

Examples of the boric acid and/or a borate ester compound include boric acid; alkyl borates such as trimethyl borate, triethyl borate, tributyl borate, tri-n-octyl borate, tri(triethylene glycol methyl ether) borate, tricyclohexyl borate, and trimethyl borate; aromatic borates such as tri-o-cresyl borate, tri-m-cresyl borate, tri-p-cresyl borate, and triphenyl borate; and others such as tri(1,3-butanediol) biborate, tri(2-methyl-2,4-pentanediol) biborate, and trioctylene glycol diborate.

The boric ester compound to use may also be a cyclic boric ester compound having a cyclic structure in its molecule. Examples of the cyclic boric ester compound include tris-o-phenylene bisborate, bis-o-phenylene pyroborate, bis-2,3-dimethylethylene pyroborate, and bis-2,2-dimethyltrimethylene pyroborate.

Of these Lewis acids, the use of a borate ester compound as compound [C2] is particularly preferable. Commercial products that contain such boric ester compounds include Cureduct (registered trademark) L-01B and L-07N (both manufactured by Shikoku Chemicals Corporation).

From the viewpoint of the balance between the preservation stability of the resin composition and the heat resistance, mechanical properties, and color of the resulting cured epoxy resin, there must exist an appropriate combination of an imidazole compound of component [B] and an acidic compound of component [C]. In particular, it is preferable to use a benzoic acid [C1] for compound [B1] and a borate ester compound [C2] for compound [B2].

<Method for Preparation of Epoxy Resin Composition>

To prepare the epoxy resin composition according to the present invention, a mixture of materials may be kneaded by using a machine such as kneader, planetary mixer, three roll mill, and twin screw extruder, or a mixture may be manually produced by using, for example, a beaker and a spatula if uniform kneading is possible. Preferred preparation methods include the following. Specifically, component [A] is put in a container and heated while stirring to an appropriate temperature in the range of 130° C. to 180° C. to ensure uniform dissolution of the epoxy resin. Subsequently, it is cooled while stirring preferably to a temperature of 100° C. or less, more preferably 80° C. or less, and still more preferably 60° C. or less, and component [B] and component [C] are fed, followed by kneading. In this instance, it is more preferred to prepare in advance a curing agent masterbatch using part of component [A] in order to ensure uniform mixing of component [B] and component [C].

<Fiber Reinforced Composite Material>

Next, details of the fiber reinforced composite material are described below. A fiber reinforced composite material containing a cured product of the epoxy resin composition according to the present invention as matrix resin can be produced by blending and integrating the epoxy resin composition according to the present invention with reinforcing fiber, followed by curing the blend

There are no specific limitations on the reinforcing fiber to be used for the present invention, and useful examples include glass fiber, carbon fiber, aramid fiber, boron fiber, alumina fiber, and silicon carbide fiber. A plurality of these fibers may be used as a mixture. Of these, the use of carbon fiber is preferred because it serves to provide a fiber reinforced composite material that is light in weight and high in rigidity and a molded article containing glossy black fibers and having good design characteristics.

The white spots on the surface of a molded article that occur when dicyandiamide is used as hardener, which constitute the major problem of the present invention, are considered to result from the filtering of dicyandiamide by fibers during the impregnation with the epoxy resin composition or the segregation of dicyandiamide in the neighborhood of fibers as the resin flows during the molding step. When dicyandiamide is used as hardener, the advantageous effect of the invention is enhanced more greatly with a decreasing single fiber diameter of the reinforcing fiber that is combined with the fiber reinforced composite material because fibers with a smaller single fiber diameter tend to suffer more white spots. From this viewpoint, it is preferable for the reinforcing fiber to have a single fiber diameter of 3 to 20 μm, more preferably 3 to 10 μm.

<Prepreg>

When producing a fiber reinforced composite material, it is preferable to preliminarily produce a prepreg composed mainly of an epoxy resin composition and reinforcing fiber in order to ensure easy storage and high handleability. Such a prepreg can be obtained by impregnating reinforcing fiber with the epoxy resin composition according to the present invention. Good techniques for the impregnation include hot melting (dry method).

Hot melting is a technique designed for direct impregnation of reinforcing fiber with an epoxy resin composition that is preliminarily heated to decrease its viscosity. Specifically, a film coated with an epoxy resin composition is first prepared on a piece of release paper or the like, and then the film is put on a sheet of paralleled reinforcing fibers or a sheet (cloth) of woven fabric reinforcing fibers from both sides or from one side thereof, and heated and pressed to ensure impregnation of the reinforcing fiber with the resin.

There are no specific limitations on the structural features of the reinforcing fiber to use for producing the prepreg, but it is preferred to use woven fabrics because they serve to produce molded articles with beautiful weave patterns and good design characteristics. In the case where dicyandiamide is used as hardener, white spots tend to be generated in the neighborhood of intersections of fibers as a woven fabric type prepreg is molded. The advantageous effect of the invention is enhanced particularly greatly when woven fabric is used as reinforcing fiber in a prepreg.

Here, there are no specific limitations on the reinforcing fiber to use in a prepreg, and the various fibers listed above in the description of fiber reinforced composite materials can be adopted. Among others, the use of carbon fiber is preferred because it serves to provide a fiber reinforced composite material that is light in weight and high in rigidity and a molded article containing glossy black fibers and having good design characteristics.

<Molding Method for Prepreg>

For laminate molding of prepreg layers, techniques that can be useful for applying heat and pressure include press molding, autoclave molding, bucking molding, wrapping tape molding, and internal pressure molding.

Fiber reinforced composite materials containing a cured product of the epoxy resin composition according to the present invention and reinforcing fiber are adopted favorably in sports applications, general industrial applications, and aerospace applications. More specifically, preferred sports applications include golf shafts, fishing rods, tennis and badminton rackets, hockey and other sticks, and skiing poles. Furthermore, preferred general industrial applications include structural and interior finishing material of vehicles (such as automobiles, motorcycles, bicycles, ships, and railroad vehicles), drive shafts, plate springs, windmill blades, pressure vessels, flywheels, rollers for paper manufacture, roofing materials, cables, and mending/reinforcing materials.

EXAMPLES

The present invention is described below in more detail with reference to Examples, but it should be understood that the invention is not construed as being limited thereto.

Determination of physical properties was performed in an environment with a temperature of 23° C. and a relative humidity of 50% unless otherwise specified.

The materials listed below were used for the preparation of various epoxy resin compositions.

<Materials Used>

Component [A]: epoxy resin

-   -   isocyanurate type epoxy resin [A1]

[A1]-1 TEPIC (registered trademark) -S (epoxy equivalent weight 100, manufactured by Nissan Chemical Industries, Ltd.)

[A1]-2 TEPIC (registered trademark) -L (epoxy equivalent weight 101, manufactured by Nissan Chemical Industries, Ltd.)

[A1]-3 TEPIC (registered trademark) -PAS B22 (epoxy equivalent weight 190, manufactured by Nissan Chemical Industries, Ltd.).

-   -   bisphenol type epoxy resin [A2]

[A2]-1 EPICLON (registered trademark) 830 (bisphenol F type epoxy resin, epoxy equivalent weight 172, manufactured by DIC Corporation)

[A2]-2 jER (registered trademark) 828 (bisphenol A type epoxy resin, epoxy equivalent weight 189, manufactured by Mitsubishi Chemical Corporation)

[A2]-3 Epotohto (registered trademark) YDF-2001 (bisphenol F type epoxy resin, epoxy equivalent weight 475, manufactured by Tohto Kasei Co., Ltd.)

[A2]-4 jER (registered trademark) 1001 (bisphenol A type epoxy resin, epoxy equivalent weight 470, manufactured by Mitsubishi Chemical Corporation)

[A2]-5 jER (registered trademark) 4004P (bisphenol F type epoxy resin, epoxy equivalent weight 910, manufactured by Mitsubishi Chemical Corporation)

[A2]-6 jER (registered trademark) 1007 (bisphenol A type epoxy resin, epoxy equivalent weight 910, manufactured by Mitsubishi Chemical Corporation)

-   -   other epoxy resins [A3]

[A3]-1 SUMI-EPDXY (registered trademark) ELM434 (diaminodiphenylmethane type epoxy resin, epoxy equivalent weight 120, manufactured by Sumitomo Chemical Co., Ltd.)

[A3]-2 jER (registered trademark) 154 (phenol novolac type epoxy resin, epoxy equivalent weight 175, manufactured by Mitsubishi Chemical Corporation)

Component [B]: Imidazole Compound

-   -   compound [B1] as represented by general formula (I)

[B1]-1 G-8009L (imidazole equivalent weight 195, compound as represented by general formula (I) where R₁ and R₂ are each an ethyl group, R₃ and R₄ are each a methyl group, and X is a hexamethylene group, manufactured by Dai-Ichi Kogyo Seiyaku Co., Ltd.)

-   -   compound [B2] as represented by general formula (II)

[B2]-1 Cureduct (registered trademark) P-0505 (imidazole equivalent weight 280, compound as represented by general formula (II) where R₅ and R₆ are each an ethyl group, R₇ and R₈ are each an methyl group, and Y is an isopropylidene group, manufactured by Shikoku Chemicals Corporation industry)

-   -   other imidazole compounds [B3]

[B3]-1 Curezol (registered trademark) 2MZ-H (imidazole equivalent weight 82, 2-methyl imidazole, manufactured by Shikoku Chemicals Corporation industry)

[B3]-2 Curezol (registered trademark) 2PZ (imidazole equivalent weight 144, 2-phenyl imidazole, manufactured by Shikoku Chemicals Corporation industry)

-   -   [B′] hardener other than imidazole compound

[B′]-1 jER Cure (registered trademark) DICY7 (dicyandiamide, manufactured by Mitsubishi Chemical Corporation)

[B′]-2 DCMU99 (3-(3,4-dichlorophenyl)-1,1-dimethylurea, manufactured by Hodogaya Chemical Co., Ltd.)

Component [C]: acidic compound

-   -   [C1] aromatic carboxylic acid

[C1]-1 benzoic acid (manufactured by Tokyo Chemical Industry Co., Ltd.)

-   -   [C2] a mixture containing a borate ester compound

[C2]-1 Curezol (registered trademark) L-01B (a mixture containing 5 parts by mass of a borate ester compound as acidic compound, manufactured by Shikoku Chemicals Corporation industry)

[C2]-2 Curezol (registered trademark) L-07N (a mixture containing 5 parts by mass of a borate ester compound as acidic compound, manufactured by Shikoku Chemicals Corporation industry)

<Methods for Calculation of Parameters of Resin Composition>

(1) Method for Calculation of the Average Epoxy Equivalent Weight of Resin [A2] in an Epoxy Resin Composition

In the case of an epoxy resin composition containing n types of epoxy resins in combination as resin [A2], with the epoxy resins in resin [A2] accounting for G′ parts by mass in total and each epoxy resin X, which has an epoxy equivalent weight of Ex (g/eq), in resin [A2] accounting for Wx parts by mass, the average epoxy equivalent weight (g/eq) of resin [A2] was calculated by Equation (I) (where x=1, 2, 3, . . . , n) given below.

$\begin{matrix} {\mspace{79mu} \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack} & \; \\ {{{Average}\mspace{14mu} {epoxy}\mspace{14mu} {equivalent}\mspace{14mu} {weight}\mspace{14mu} {of}\mspace{14mu} {{resin}\mspace{14mu}\left\lbrack {A\; 2} \right\rbrack}\mspace{14mu} {in}\mspace{14mu} {epoxy}\mspace{14mu} {resin}\mspace{14mu} {composition}\mspace{14mu} \left( {g\text{/}{eq}} \right)} = \frac{G^{\prime}}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & (I) \end{matrix}$

(2) Method for Calculation of Average Epoxy Equivalent Weight Over all Epoxy Resins in Epoxy Resin Composition

In the case of an epoxy resin composition containing a combination of n types of epoxy resins as component [A] and also containing epoxy resins accounting for G parts by mass in total, with each epoxy resin Y, which has an epoxy equivalent weight of Ey (g/eq), accounting for Wy parts by mass, the average epoxy equivalent weight (g/eq) over all epoxy resins is calculated by Equation (II) (where y=1, 2, 3, . . . , n) given below.

$\begin{matrix} {\mspace{79mu} \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack} & \; \\ {{{Average}\mspace{14mu} {epoxy}\mspace{14mu} {equivalent}\mspace{14mu} {weight}\mspace{14mu} {over}\mspace{14mu} {all}\mspace{14mu} {epoxy}\mspace{14mu} {{resin}s}\mspace{25mu} {in}\mspace{14mu} {epoxy}\mspace{14mu} {resin}\mspace{14mu} {composition}\mspace{14mu} \left( {g\text{/}{eq}} \right)} = \frac{G}{\left\{ {\left( \frac{W_{1}}{E_{1}} \right) + {\left( \frac{W_{2}}{E_{2}} \right)\ldots} + {\left( \frac{W_{x}}{E_{x}} \right)\ldots} + \left( \frac{W_{n}}{E_{n}} \right)} \right\}}} & ({II}) \end{matrix}$

(3) Method for Calculation of Ratio of the Number of Imidazole Groups to the Number of Epoxy Groups in Epoxy Resin Composition

In the case of an epoxy resin composition containing epoxy resins with a total quantity accounting for G parts by mass and also containing an imidazole compound accounting for W parts by mass in the epoxy resin composition and having an imidazole equivalent weight of I (g/eq), the ratio of the number of imidazole groups to the number of epoxy groups in the epoxy resin composition was calculated by Equation (III) from the value obtained in the above paragraph (2).

[Formula 6]

Ratio of the number of imidazole groups to the number of epoxy groups in epoxy resin composition=average epoxy equivalent weight of epoxy resin composition×W/(G×I)  (III)

<Method for Preparation of Epoxy Resin Composition>

(1) Preparation of Curing Agent Masterbatch

A liquid-state bisphenol type epoxy resin [A2] ([A2]-1 and/or [A2]-2 to be contained in the resin composition) was prepared in an amount of 10 parts by mass (accounting for 10 parts by mass relative to the total quantity of epoxy resins in component [A], which accounts for 100 parts by mass). To this, an imidazole compound for component [B], a hardener [B′] other than the imidazole compound, and an acidic compound for component [C] that are to be contained in the resin composition are added and kneaded in a kneader at room temperature. The resulting mixture was passed through a three roll mill twice to prepare a curing agent masterbatch.

(2) Preparation of Epoxy Resin Composition

Excluding the 10 parts by mass of liquid-state bisphenol type epoxy resin [A2] used in paragraph (1) above, 90 parts by mass of the epoxy resin of component [A] is put in a kneader. While kneading, the temperature was raised to 150° C. and it was maintained for 1 hour to provide a transparent viscous liquid. After cooling the liquid to 60° C. while continuing to knead it, the curing agent masterbatch prepared in paragraph (1) above was added, followed by kneading at the temperature for 30 minutes to prepare an epoxy resin composition. Tables 1 to 4 show the components of the epoxy resin compositions prepared in each Example and Comparative example.

<Method for Production of Cured Epoxy Resin>

An epoxy resin composition prepared according to the <Method for preparation of epoxy resin compositions> described above was defoamed in a vacuum and cured at a temperature of 130° C. for 90 minutes in a mold set for a thickness of 2 mm using a 2 mm thick spacer of Teflon (registered trademark), thus providing a cured epoxy resin plate with a thickness of 2 mm.

<Method for Production of Woven Fabric Carbon Fiber Reinforced Composite Material (Hereinafter, Woven Fabric CFRP)>

An epoxy resin composition prepared according to the <Method for preparation of epoxy resin composition> described above was spread on a piece of release paper using a film coater to produce a resin film having a metsuke of 66 g/m². A piece of two-directional carbon fiber cloth (2/2 twill weave, metsuke 198 g/m²) (Torayca (registered trademark) T300, manufactured by Toray Industries, Inc.) was sandwiched between two sheets of resin film and heat-compressed in a prepregging apparatus to ensure impregnation from both sides to provide a woven fabric prepreg. The resin accounted for 40 mass % of the prepreg.

Ten plies of this woven fabric prepreg were laid up with their fibers aligned in the same direction, covered with a nylon film in a gapless manner, and subjected to heat-compression molding in an autoclave at 130° C. for 2 hours under an internal pressure of 0.3 MPa to ensure curing to prepare a woven fabric CFRP.

<Methods for Evaluation of Physical Properties>

(1) Preservation Stability of Epoxy Resin Composition

The preservation stability of an epoxy resin composition is evaluated in terms of the variation in Tg that is determined as described below. In a container having a round bottom with a diameter of 4 cm, 2 g of an epoxy resin composition prepared according to the <Method for preparation of epoxy resin composition> described above was fixed and it was stored for 7 days in a constant temperature and humidity tank placed in an environment with a temperature of 25° C. and a relative humidity of 50% RH. A 3 mg sample of the resin was weighed on a pan before and after the storage and measurements were taken using a differential scanning colorimeter (Q-2000, manufactured by TA Instrument) while increasing the temperature from −50° C. to 100° C. at a constant heating rate of 10° C./minute. The midpoint in the inflection section of the resulting heat-temperature curve was taken as the glass transition temperature (hereinafter denoted as Tg). The Tg variation was calculated by subtracting the Tg value measured before the storage from the Tg value measured after the storage. A smaller Tg variation shows a better preservation stability.

(2) Tg of Cured Epoxy Resin

A test piece with a width of 10 mm, a length of 40 mm, and a thickness of 2 mm was cut out of a cured epoxy resin sample prepared according to the <Method for production of cured epoxy resin> described above and subjected to measurement by a dynamic viscoelasticity measuring apparatus (DMA-Q800, manufactured by TA Instruments) under the conditions of a deformation mode of cantilever bending, a span of 18 mm, a strain of 20 μm, a frequency of 1 Hz, and a constant temperature raising rate of 5° C./min for heating from 40° C. to 200° C. Tg was determined as the onset temperature of storage elastic modulus in the resulting storage elastic modulus-temperature curve.

(3) Elastic Modulus of Cured Epoxy Resin

A test piece having a width of 10 mm and a length of 60 mm was cut out of a cured epoxy resin sample prepared according to the <Method for production of cured epoxy resin> described above and subjected to three-point bending test using an Instron type universal tester (manufactured by Instron Corporation) with a span of 32 mm and a crosshead speed of 100 mm/minute according to JIS K7171 (1994) to measure the elastic modulus. Measurements were taken from 6 specimens (n=6) and the average was adopted as their elastic modulus.

(4) Yellowness Index of Cured Epoxy Resin

A 3 cm×3 cm test piece with a thickness of 2 mm was cut out of a cured epoxy resin sample prepared according to the <Method for production of cured epoxy resin> described above. This test piece was subjected to measurement of the color of transmitting objects using a spectrophotometer (MSC-P, manufactured by Suga Test Instruments Co., Ltd.) according to JIS Z8722 (2009) to determine the tristimulus values. A D65 Illuminant was used under the geometrical condition of e, and measurements were taken by spectrophotometric colorimetry with an effective wavelength width of 5 nm and wavelength intervals of 5 nm. The color system used was the XYZ color system. Based on the tristimulus values thus obtained, the yellowness index was calculated according to JIS K7373 (2006).

(5) Appearance of Woven Fabric CFRP

Woven fabric CFRP was prepared according to the <Method for production of woven fabric CFRP> described above and immersed in 40° C. water for 7 days. After the immersion, the appearance of the woven fabric CFRP was observed visually focusing on the portions around intersections of fibers. A specimen was rated as good when no white spots were found and rated as poor when white spots were found.

Example 1

An epoxy resin composition was prepared according to the <Method for preparation of epoxy resin composition> described above using 20 parts by mass of TEPIC (registered trademark) -L, 25 parts by mass of jER (registered trademark) 828, and 55 parts by mass of Epotohto (registered trademark) YDF2001 as the epoxy resins of component [A], 3 parts by mass of G-8009L as the imidazole compound of component [B], and 1 part by mass of benzoic acid as the acidic compound of component [C].

For this epoxy resin composition, the Tg variation was measured and found to be +4° C., which shows a high preservation stability.

From the epoxy resin composition obtained, cured epoxy resin was prepared according to the <Method for production of cured epoxy resin>. The Tg, flexural modulus, and yellowness index of this cured epoxy resin were measured and results showed a Tg of 135° C., a flexural modulus of 3.5 GPa, and a yellowness index of 54, which suggest that the cured resin had good physical properties. Then, woven fabric CFRP was prepared from the epoxy resin composition obtained and its appearance was observed. The observation showed no white spots.

Examples 2 to 19

Except for using the resin components that are shown in Tables 1 and 2, the same procedure as in Example 1 was carried out to prepare epoxy resin compositions, cured epoxy resins, and woven fabric CFRPs.

The preservation stability of the epoxy resin composition, the Tg, elastic modulus, and yellowness index of the cured epoxy resins, and the appearance of the woven fabric CFRP determined in each Example are given in Tables 1 and 2, showing that good results were obtained for all of them.

Example 20

Except for using a resin component free of acidic compound as shown in Table 2, the same procedure as in Example 1 was carried out to prepare an epoxy resin composition, cured epoxy resin, and woven fabric CFRP. The epoxy resin composition obtained was slightly lower in preservation stability than the one in Example 1, but it was almost as good as the one in Example 1 with respect to the other physical properties evaluated.

Comparative Example 1

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Results of evaluation of physical properties are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the Tg and elastic modulus of the cured epoxy resin, and the appearance of the woven fabric CFRP, but resin [A2] accounted for only less than 40 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had an undesirable yellowness index.

Comparative Example 2

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the Tg and elastic modulus of the cured epoxy resins and the appearance of the woven fabric CFRP, but the epoxy resin composition had a slightly low preservation stability. In addition, resin [A2] accounted for only less than 40 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had an undesirable yellowness index.

Comparative Example 3

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the Tg and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but resin [A1] accounted for only less than 10 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had a small elastic modulus.

Comparative Example 4

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the Tg and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but resin [A1] accounted for only less than 10 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had a small elastic modulus.

Comparative Example 5

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the Tg and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but resin [A1] accounted for only less than 10 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had a small elastic modulus.

Comparative Example 6

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the Tg and elastic modulus of the cured epoxy resin, and the appearance of the woven fabric CFRP, but resin [A1] accounted for more than 40 parts by mass relative to the total quantity of epoxy resins, which accounted for 100 parts by mass, and the cured epoxy resin had an undesirable yellowness index.

Comparative Example 7

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 3. Evaluation results are also shown in Table 3. Good results were obtained for the preservation stability of the epoxy resin composition, the elastic modulus and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but the average epoxy equivalent weight of resin [A2] failed to reach 220 g/eq, and the cured epoxy resin had a low Tg.

Comparative Example 8

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the preservation stability of the epoxy resin composition, the elastic modulus and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but the average epoxy equivalent weight of resin [A2] exceeded 500 g/eq, and the cured epoxy resin had a low Tg.

Comparative Example 9

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the preservation stability of the epoxy resin composition, the elastic modulus and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but the content of component [B] was small, and the ratio of the number of imidazole groups to the number of epoxy groups was less than 0.01, resulting in cured epoxy resin with a low Tg.

Comparative Example 10

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the Tg and elastic modulus of the cured epoxy resin and the appearance of the woven fabric CFRP, but the content of component [B] was large and the ratio of the number of imidazole groups to the number of epoxy groups was more than 0.06. As a result, the epoxy resin composition had a low preservation stability and the cured epoxy resin had an undesirable yellowness index.

Comparative Example 11

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the preservation stability of the epoxy resin composition, the elastic modulus and yellowness index of the cured epoxy resin, and the appearance of the woven fabric CFRP, but component [B] was not contained, and the cured epoxy resin had a low Tg.

Comparative Example 12

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the preservation stability of the epoxy resin composition and the Tg, elastic modulus, and yellowness index of the cured epoxy resin, but component [B] was not contained whereas dicyandiamide was contained, resulting in white spots being found in the woven fabric CFRP.

Comparative Example 13

An epoxy resin composition, cured epoxy resin, and woven fabric CFRP were prepared by the same procedure as in Example 1 using the resin components given in Table 4. Evaluation results are also shown in Table 4. Good results were obtained for the Tg of the cured epoxy resin and the appearance of the woven fabric CFRP, but component [A1] was not contained, and the cured epoxy resin had a small elastic modulus. In addition, the content of component [B] was large and the ratio of the number of imidazole groups to the number of epoxy groups was more than 0.06. As a result, the epoxy resin composition had an undesirable preservation stability and the cured epoxy resin had an undesirable yellowness index.

TABLE 1 Example Example Example Example Example Components 1 2 3 4 5 Epoxy resin [A] [A1] TEPIC ® -S composition epoxy resin isocyanuric acid TEPIC ® -L 20 20 10 25 40 (parts by type epoxy resin TEPIC ® -PAS B22 mass) [A2] EPICLON ® 830 20 20 20 bisphenol type jER ® 828 25 15 epoxy resin Epotohto ® 55 33 YDF2001 jER ® 1001 70 55 40 jER ® 4004P jER ® 1007 [A3] other jER  154 35 epoxy resin [B] [B1] G-8009L 3 3 3 3 3 imidazole compound compound represented by general formula (I) [B2] Curezol ® P-0505 compound represented by general formula (II) [B3] Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoic acid 1 1 1 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ® L-07N mixture containing borate ester compound Resin average epoxy equivalent 322 322 341 323 299 composition weight of resin [A2] g · eq⁻¹ parameter average epoxy equivalent weight 224 184 276 209 168 over all epoxy resins g · eq⁻¹ ratio of imidazole groups to 0.034 0.028 0.042 0.032 0.026 epoxy groups Physical epoxy resin preservation stability 4 4 4 4 4 properties composition (Tg variation ° C.) evaluation cured epoxy Tg ° C. 135 136 125 133 144 results resin elastic modulus GPa 3.5 3.5 3.3 3.5 3.7 yellowness index 54 77 58 55 72 woven appearance (white spot good good good good good fabric absent: good, CFRP white spot existent: poor) Example Example Example Example Example Components 6 7 8 9 10 Epoxy resin [A] [A1] TEPIC ® -S 20 20 20 20 composition epoxy resin isocyanuric acid TEPIC ® -L (parts by type epoxy resin TEPIC ® -PAS B22 20 mass) [A2] EPICLON ® 830 40 15 15 bisphenol type jER ® 828 25 25 epoxy resin Epotohto ® 55 55 YDF2001 jER ® 1001 40 55 45 jER ® 4004P 10 jER ® 1007 20 [A3] other jER  154 epoxy resin [B] [B1] G-8009L 3 3 imidazole compound compound represented by general formula (I) [B2] Curezol ® P-0505 3 3 3 compound represented by general formula (II) [B3] Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoic acid 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ® L-07N 1 1 1 mixture containing borate ester compound average epoxy equivalent 322 322 253 374 417 weight of resin [A2] g · eq⁻¹ average epoxy equivalent weight 223 283 194 242 255 over all epoxy resins g · eq⁻¹ ratio of imidazole groups to 0.034 0.044 0.021 0.026 0.027 epoxy groups Physical epoxy resin preservation stability 4 4 3 3 3 properties composition (Tg variation ° C.) evaluation cured epoxy Tg ° C. 134 133 128 139 127 results resin elastic modulus GPa 3.5 3.3 3.5 3.5 3.5 yellowness index 63 78 54 41 34 woven appearance (white spot good good good good good fabric absent: good, CFRP white spot existent: poor)

TABLE 2 Example Example Example Example Example Component 11 12 13 14 15 Epoxy [A] [A1] TEPIC ® -S 30 30 30 resin epoxy isocyanuric TEPIC ® -L 20 20 composition resin acid type TEPIC ® (parts by epoxy resin -PAS B22 mass) [A2] EPICLON ® 830 bisphenol type jER ® 828 25 25 25 25 25 epoxy resin Epotohto ® 45 45 45 55 55 YDF2001 jER ® 1001 jER ® 4004P jER ® 1007 [A3] other jER ® 154 epoxy resin [B] [B1] G-8009L imidazole compound compound represented by general formula (I) [B2] Curezol ® P-0505 2 3 8 compound represented by general formula (II) [B3] Curezol ® 2MZ-H 1.3 other imidazole Curezol ® 2PZ 2.2 compound [C] [C1] benzoic acid 1 1 acidic aromatic compound carboxylic acid [C2] Curezol ® L-07N 1 1 1 mixture containing borate ester compound Resin average epoxy equivalent 308 308 308 322 322 composition weight of resin [A2] g · eq⁻¹ average epoxy equivalent weight 190 190 190 224 224 over all epoxy resins parameter g · eq⁻¹ ratio of imidazole groups 0.014 0.020 0.054 0.036 0.034 to epoxy groups Physical epoxy resin preservation stability 0 3 7 4 4 properties composition (Tg variation ° C.) evaluation cured Tg ° C. 128 137 148 130 132 results epoxy elastic modulus GPa 3.5 3.5 3.5 3.5 3.5 resin yellowness index 27 41 70 62 67 woven appearance (white spot good good good good good fabric absent: good, CFRP white spot existent: poor) Example Example Example Example Example Component 16 17 18 19 20 Epoxy [A] [A1] TEPIC ® -S resin epoxy isocyanuric TEPIC ® -L 20 20 20 20 20 composition resin acid type TEPIC ® -PAS (parts by epoxy resin B22 mass) [A2] EPICLON ® 830 bisphenol type jER ® 828 25 25 25 25 25 epoxy resin Epotohto ® 55 55 55 55 55 YDF2001 jER ® 1001 jER ® 4004P jER ® 1007 [A3] other jER ® 154 epoxy resin [B] [B1] G-8009L 3 imidazole compound compound represented by general formula (I) [B2] Curezol ® P-0505 4.3 3 3 3 compound represented by general formula (II) [B3] Curezol ® 2MZ-H other imidazole Curezol ® 2PZ compound [C] [C1] benzoic acid 1 acidic aromatic compound carboxylic acid [C2] mixture Curezol ® L-07N 0.5 3 6 containing borate ester compound Resin average epoxy equivalent 322 322 322 322 322 composition weight of resin [A2] g · eq⁻¹ average epoxy equivalent weight 224 224 224 224 224 over all epoxy resins parameter g · eq⁻¹ ratio of imidazole groups 0.034 0.034 0.034 0.034 0.034 to epoxy groups Physical epoxy resin preservation stability 4 5 3 1 11 properties composition (Tg variation ° C.) evaluation cured Tg ° C. 138 136 133 131 135 results epoxy elastic modulus GPa 3.5 3.5 3.5 3.5 3.5 resin yellowness index 40 52 56 51 59 woven appearance (white spot good good good good good fabric absent: good, CFRP white spot existent: poor)

TABLE 3 Compar- Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative ative example example example example example example example Components 1 2 3 4 5 6 7 Epoxy [A] [A1] TEPIC ® -L 20 20 50 20 resin epoxy isocyanuric compo- resin acid type sition epoxy resin (parts by [A2] EPICLON ® 20 80 mass) bisphenol 830 type jER ® 828 10 10 31 25 25 epoxy Epotohto ® 22 22 69 55 55 resin YDF2001 jER ® 1001 30 jER ® 1007 [A3] SUMI- 20 other EPOXY ® epoxy ELM 434 resin JER ® 154 48 48 20 [B] [B2] Curezol ® 3 3 3 3 3 3 3 imida- compound P-0505 zole represented com- by general pound formula (II) [B3] Curezol ® other 1B2P imidazole compound [B’] JER Cure ® hardener DICY7 other than DCMU99 imidazole compound [C] [C2] Curezol ® acidic mixture L-01B com- containing Curezol ® 1 1 1 1 1 1 pound borate ester L-07N compound Resin average epoxy equivalent 322 322 323 322 322 279 172 compo- weight of resin [A2] g · eq⁻¹ sition average epoxy equivalent weight 176 176 323 241 277 148 151 para- over all epoxy resins meter parameter g · eq⁻¹ ratio of imidazole groups 0.019 0.019 0.035 0.026 0.030 0.016 0.016 to epoxy groups Character- epoxy preservation stability 4 10 4 4 4 4 4 istics resin (Tg variation ° C.) evalu- compo- Tg ° C. 138 137 129 127 127 137 112 ation sition elastic modulus GPa 3.5 3.5 3.0 3.1 3.1 3.7 3.5 results cured yellowness index 100 98 26 79 61 101 68 epoxy appearance good good good good good good good resin (white spot absent: woven good, white fabric spot existent: poor) CFRP

TABLE 4 Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative Components example 8 example 9 example 10 example 11 example 12 example 13 Epoxy [A] [A1] TEPIC ® -L 20 20 20 20 20 resin epoxy isocyanuric composition resin acid type (parts by epoxy resin mass) [A2] EPICLON ® 830 20 25 25 25 25 bisphenol type jER ® 828 70 epoxy resin Epotohto ® YDF2001 jER ® 1001 55 55 55 55 jER ® 1007 60 [A3] SUMI-EPOXY ® other epoxy ELM434 resin JER ® 154 30 [B] [B2] Curezol ® P-0505 3 1 8 20 imidazole compound compound represented by general formula (II) [B3] JER ® 1B2PZ other imidazole compound [B’] JER Cure ® DICY7 6 hardener DCMU99 6 3 other than imidazole compound [C] [C2] Curezol ® L-01B 11 acidic mixture Curezol ® L-07N 1 1 1 compound containing borate ester compound Resin average epoxy equivalent 545 306 306 306 306 189 compo- weight of resin [A2] g · eq⁻¹ sition average epoxy equivalent weight 290 218 218 218 218 186 parameter over all epoxy resins g · eq⁻¹ ratio of imidazole groups 0.031 0.008 0.062 — — 0.133 to epoxy groups Character- epoxy preservation stability 4 1 11 4 4 10 resin (Tg variation ° C.) istics compo- Tg ° C. 111 100 149 105 124 143 evalu- sition elastic modulus GPa 3.5 3.5 3.5 3.5 3.5 3.1 ation cured yellowness index 27 29 126 73 30 151 results epoxy appearance good good good good poor good resin (white spot woven absent: good, white fabric spot existent: poor) CFRP

The epoxy resin composition according to the present invention serves to produce a cured epoxy resin that simultaneously realizes a high heat resistance, a high elastic modulus, and a low color and therefore, fiber reinforced composite materials containing this as matrix resin have a high heat resistance, good mechanical properties, and a low color. Furthermore, molded articles produced from such fiber reinforced composite materials do not suffer the formation of white spots on the surface thereof, and this feature, in combined with their low color feature, can ensure high designability. The epoxy resin composition, prepreg, and fiber reinforced composite material according to the present invention can be applied favorably to sport applications and general industrial applications. 

1. An epoxy resin composition comprising an epoxy resin as component [A] and an imidazole compound as component [B] and meeting the undermentioned conditions (a) to (d): (a) the epoxy resin composition contains an isocyanurate type epoxy resin [A1] as component [A] in an amount of 10 to 40 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (b) the epoxy resin composition contains an bisphenol type epoxy resin [A2] as a component [A] in an amount of 40 to 90 parts by mass relative to the total quantity of epoxy resins which represents 100 parts by mass, (c) resin [A2] has an average epoxy equivalent weight of 220 to 500 g/eq, and (d) the content of component [B] is such that the ratio of the number of imidazole groups to the total number of epoxy groups in the epoxy resins is 0.01 to 0.06.
 2. An epoxy resin composition as set forth in claim 1 wherein component [B] has an imidazole equivalent weight of 180 g/eq or more.
 3. An epoxy resin composition as set forth in either claim 1 wherein a compound [B1] as represented by general formula (I) is contained as component [B].

wherein R₁, R₂, R₃ and R₄ are each independently a hydrogen atom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, or a phenyl group, and X is an alkylene group or an aromatic hydrocarbon group.
 4. An epoxy resin composition as set forth in claim 1 wherein a compound [B2] as represented by general formula (II) is contained as component [B].

wherein R₅, R₆, R₇ and R₈ are each independently a hydrogen atom, an aliphatic hydrocarbon group with a carbon number of 1 to 20, or a phenyl group, and Y is a single bond, an alkylene group, an alkylidene group, ether group, or sulfonyl group.
 5. An epoxy resin composition as set forth in claim 1 further comprising an acidic compound as component [C].
 6. An epoxy resin composition as set forth in claim 5 comprising an aromatic carboxylic acid [C1] as component [C].
 7. An epoxy resin composition as set forth in claim 5 comprising a borate ester compound [C2] as component [C].
 8. An epoxy resin composition as set forth in claim 1 wherein dicyandiamide accounts for 0.5 part by mass or less relative to the total quantity of epoxy resins which accounts for 100 parts by mass.
 9. A prepreg comprising an epoxy resin composition as set forth in claim 1 and reinforcing fiber.
 10. A prepreg as set forth in claim 9 wherein the reinforcing fiber is in the form of woven fabric.
 11. A prepreg as set forth in claim 9 wherein the reinforcing fiber is carbon fiber.
 12. A fiber reinforced composite material obtainable by curing a prepreg as set forth in claim
 11. 